Introduce Group level setting for Delayed Project removal

What does this MR do?

This MR adds a Group level Setting delayed_project_removal to configure whether Projects within the scope of the Group should be deleted after a delayed interval, or immediately.

Group admin can configure this Setting under Settings -> General -> Permissions, LFS, 2FA menu (ref screenshot below).

• When the Setting is enabled, projects within the groups will be deleted after the delayed interval defined in Application Setting.

• When the Setting is disabled, projects within the groups will be deleted immediately.

The Setting is disabled by default for all groups.

All user namespace projects are deleted immediately.

Feature flag

The option to let Groups to configure this Setting is gated behind the Feature flag configure_project_deletion_mode. It is disabled by default.

The enablement of the flag depends on !36461 (merged)

Test plan

Feature flag enabled?	Use case	Expected behavior	Status
true	Project in user namespace	Deletes immediately
true	delayed_project_removal disabled for group	Deletes immediately
true	delayed_project_removal enabled for group	Deletes after delayed period
false	Project in user namespace	Deletes after delayed period
false	Project in a group	Deletes after delayed period

Mentions #220382 (closed)

Screenshots

Database output

Migration output:

== 20200701190523 AddDelayedProjectRemovalToNamespaces: migrating =============
-- add_column(:namespaces, :delayed_project_removal, :boolean, {:default=>false, :null=>false})
   -> 0.0053s
== 20200701190523 AddDelayedProjectRemovalToNamespaces: migrated (0.0089s) ====

Rollback output:

== 20200701190523 AddDelayedProjectRemovalToNamespaces: reverting =============
-- remove_column(:namespaces, :delayed_project_removal)
   -> 0.0016s
== 20200701190523 AddDelayedProjectRemovalToNamespaces: reverted (0.0084s) ====

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team